GTA4 is more dependent on the number of processor cores, rather than the video card.
I remember seeing a review(can't find it now, hmm...) which said there was not a big difference between using a 9800gt or a GTX 280, but there was a significant improvement going from a dual core to a quad core.
MMM Tasty GTA IV Benchmark - Overclock.net - Overclocking.net
I remember seeing a review(can't find it now, hmm...) which said there was not a big difference between using a 9800gt or a GTX 280, but there was a significant improvement going from a dual core to a quad core.
MMM Tasty GTA IV Benchmark - Overclock.net - Overclocking.net